On Thu, Nov 08, 2012 at 04:52:33PM -0800, Sonny Rao wrote:
> The system uses global_dirtyable_memory() to calculate
> number of dirtyable pages/pages that can be allocated
> to the page cache. A bug causes an underflow thus making
> the page count look like a big unsigned number. This in turn
> confuses the dirty writeback throttling to aggressively write
> back pages as they become dirty (usually 1 page at a time).
>
> Fix is to ensure there is no underflow while doing the math.
Good catch, thanks!
> Signed-off-by: Sonny Rao <sonny...@chromium.org>
> Signed-off-by: Puneet Kumar <puneets...@chromium.org>
> ---
> v2: added apkm's suggestion to make the highmem calculation better
> mm/page-writeback.c | 17 +++++++++++++++--
> 1 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 830893b..ce62442 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -201,6 +201,18 @@ static unsigned long highmem_dirtyable_memory(unsigned
> long total)
> zone_reclaimable_pages(z) - z->dirty_balance_reserve;
> }
> /*
> + * Unreclaimable memory (kernel memory or anonymous memory
> + * without swap) can bring down the dirtyable pages below
> + * the zone's dirty balance reserve and the above calculation
> + * will underflow. However we still want to add in nodes
> + * which are below threshold (negative values) to get a more
> + * accurate calculation but make sure that the total never
> + * underflows.
> + */
> + if ((long)x < 0)
> + x = 0;
> +
> + /*
> * Make sure that the number of highmem pages is never larger
> * than the number of the total dirtyable memory. This can only
> * occur in very strange VM situations but we want to make sure
> @@ -222,8 +234,9 @@ static unsigned long global_dirtyable_memory(void)
> {
> unsigned long x;
>
> - x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages() -
> - dirty_balance_reserve;
> + x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
> + if (x >= dirty_balance_reserve)
> + x -= dirty_balance_reserve;
That can be converted to "if ((long)x < 0) x = 0;", too.
And I suspect zone_dirtyable_memory() needs similar fix, too.
Thanks,
Fengguang
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